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AS-201 Weather Forecasting. This section discusses: Various weather forecasting methods, their tools, and forecasting accuracy and skill Images for the forecasting of 6 cities Applications of weather forecasting. Four forecasting steps:.

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AS-201 Weather Forecasting

This section discusses:

Various weather forecasting methods, their tools, and forecasting accuracy and skill

The future conditions here will be like weather that historically occurred when similar conditions were present

Climatology

Future conditions here will be near the monthly average for here

Numerical Weather Prediction

Future conditions here will be as predicted by supercomputers running equations of atmospheric motion

“Reading the Sky”

Future conditions here can be predicted by using current conditions and forecasting rules of thumb (ROT)

Persistence Forecast The persistence method works well when weather patterns change very little and features on the weather maps move very slowly. It also works well in places like southern California, where summertime weather conditions vary little from day to day. Trend Forecasting

Example: frontal movement

Analogue Forecasting Method—Regime Forecasting Figure 14.7 Climatology Climatology--winds Probability Forecasts Figure 14.6 Climate records can be used to generate probability forecasts for a given event. Florida has less than a 5% chance of snow on December 25th, while northern Minnesota has had snow on that date for each of the past 30 years. At 00Z and 12Z each day, data is collected and NWP models are run NWP is the 2nd largest user of super computers in the world Numerical Weather Prediction (NWP) NWP--Limitations are run Local Weather “signs” are run

Understanding how the atmosphere works (“red sky at morning…”, halo around the moon, clear skies at sunset, etc, etc, etc)

Local rules of thumb (ROT) (for example--southerly flow in March)

Table 14.3, page 378; and appendix E (page A11)

If you have good “situational awareness” of the atmosphere, rudimentary wx measurements (temp, DP, pressure, winds), and good ROT—you can make a pretty good 0-24 hour forecast.

Weekly & Monthly Forecasts are run Stationary weather systems often allow for trend based extended weather forecasts, while multiple runs of numerical weather models, known as ensemble forecasts, allow for 30 to 90 day outlooks. Figure 14.8A Figure 14.8B Numerical weather models are run multiple times with small changes in the initial conditions Gives forecaster a good estimate of the possible range of weather conditions at a future times Ensemble Modeling Accuracy and skill changes in the initial conditions

Skill—how complicated/hard is the forecast (Southern California in summer…)?

Forecast skill is determined by comparing your forecast against either persistence or climatology

Longer-range climatological forecasting improving

Tornadoes…where they are likely to form: 3 days in advance, but precise area struck is forecastable minutes-few hours in advance

Forecast Product Accuracy changes in the initial conditions Tools of the Trade changes in the initial conditions Analysis to Prognosis--NWP changes in the initial conditions Figure 14.1A Figure 14.1B Two forecast model 500 mb progs for the same time. Forecast models lose accuracy over time due to errors in the model, chaos in the atmosphere, and measurement errors Data Processing and Display changes in the initial conditions Figure 14.2 Viewing weather images, overlays, and graphs in multiple windows is done using the National Weather Service's Advanced Weather Interactive Processing System (AWIPS), which gathers data from the Automated Surface Observing System among other sources. WSR-88D Doppler Radar changes in the initial conditions Weather Surveillance Radar - 1988 Doppler, also known as next generation radar (NEXRAD), detects severe weather size, movement, and intensity. Data received by the NEXRAD unit are processed by algorithms to assist the forecaster in weather interpretation. Figure 14.3 Meteogram Display changes in the initial conditions Predicted trends in several weather variables are plotted for a 60 hour period on a meteogram. Patterns in variable response, such as rising pressure and a stop in precipitation, are readily observed. Figure 14.4 Probability of Precipitation (PoP) changes in the initial conditions describes the chance of an event occurring at any point in the forecast area, over a certain period of time (usually 12 hours). PoP is defined as: PoP = Pa x Pc, where: Pa =the probability that precipitation will occur somewhere in the forecast area during the forecast period and Pc =the percent of the area that will receive measurable precipitation, if it does occur NWS Probability of Precipitation Vertical Sounding Profile changes in the initial conditions Radiosonde instruments attached to pilot balloons are launched twice daily to profile weather variables with height. This example shows winds veering (clockwise—warm advection is occurring) from easterly at the surface to southwesterly aloft that may change the freezing rain in the saturated lower atmosphere to non-freezing rain. Figure 14.5 Vertical soundings are also used for strength and timing of convective wx, cloud types and heights, turbulence and icing, etc. Forecast Practice changes in the initial conditions Scenarios Surface Chart Predictions changes in the initial conditions

- 3-hour pressure tendencies plotted on isallobar maps help predict the movement of highs and lows

Lows tend to move toward the region of greatest pressure fall, while highs move toward the region of greatest rise.

Direct costs due to weather on airline operations can be separated into several categories: diversion, cancellation, delay and insurance. The cost of a diverted flight can be as high as $150,000 and a cancellation close to $40,000 (Irrgang and McKinney, 1992).

A report from the Air Transport Association (ATA) states that the direct annual costs to sixteen member airlines of the first two categories listed above are $47 million and $222 million, respectively (Air Traffic Management in the Future Air Navigation System, 1994).

improved forecasts of tropical weather conditions (wind, waves, disturbances) can reduce delays in drilling operations at a cost of up to $250,000 per rig per day (several thousand rigs in the Gulf).

improved hurricane track predictions could reduce days of production shutdown, each day of which costs the industry and the U.S. treasury a combined $15,000,000.

Vegetable processing:

improved temperature and precipitation forecasts can lead to greater efficiency in chemical spraying (e.g., pesticides), which costs $10-$15 per acre per application for hundreds of thousands of acres.

on a national scale the annual cost of lost production to the vegetable processing industry, primarily due to weather, is $42,500,000.

Insurance:

a single hurricane could lead to more than $50,000,000,000 in damages.

weather-related catastrophes have led to more than $48,000,000,000 in property insurance claims over the period 1989-1993.

Wx Impacts on Economy IMC (Instrument Meteorological Conditions)

Rail transportation:

it costs $2,000 per hour to stop a train. A single tornado warning covering 15 miles of track for 15 minutes can lead to seven stopped trains.

Persistence forecast is a prediction that future wx will be the same as the present.

The trend method is based on the assumption that systems continue presently movement

The analogue method (regimes) makes a wx prediction by comparing past wx maps and wx patterns to those of the present.

Climatological forecasts are based on the climatology (average weather) of a particular region

NWP is a useful tool for forecasting, but has limitations

Con’t IMC (Instrument Meteorological Conditions)

Ensemble forecasting is a technique based on running several forecast models, each beginning with slightly different weather information to reflect errors in the measurements. If they agree—forecaster can have a high confidence in the model forecast.

For a forecast to show skill, it must be better than a persistence forecast or climatological forecast

After a number of days, the atmosphere’s chaotic behavior, along with flaws in computer models and small errors in measurements, limits the accuracy of forecast models

Surface lows tend to move in direction parallel to the isobars (in the warm sector), and toward the direction of greatest pressure fall (isoallobaric)

Con’t IMC (Instrument Meteorological Conditions)

Weather conditions have a huge effect on commercial and civil aviation

Most GA crashes occur in IMC conditions

The majority of flight delays in the National Airspace System (NAS) are due to weather

Weather can have a significant impact on the economy—resulting in billions of dollars in lost revenues; and can kill thousands

Companies are learning to “own the weather” to minimize losses and increase competitiveness

The US armed forces introduced “own the weather” concepts now in use throughout the meteorological community

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